STEMS 241 



is the bark cut through, but the young wood is cut into. 



This interferes with the movement of water up the stem as 



well as with conduction by the sieve-vessels. If a small 



portion of the bark is removed, the incision extending only 



to the wood, as in the making of inscriptions on trees, the 



wound is healed, unless too large, by the growth of tissue 



from all sides until it is closed over. In this new tissue a cork 



cambium is developed, 



and presently there may 



be no surface indication 



of the wound. But if 



the wound has gone 



deeper and entered the 



wood, the record of it 



may always be found in 



the wood by removing 



the bark. In this way 



old inscriptions have 



often been uncovered. 



The well-known opera- 

 tion of grafting depends 

 upon the ability of plants 

 to heal wounds. The 

 plant upon which the 



. . FIG. 209. Cleft-grafting, showing scions in 



Operation IS performed place (A) and the wound sealed with clay 



is called the stock, and 



the twig grafted into it the scion. An ordinary method, called 

 cleft-grafting, is to cut off the stem or a branch of the stock, 

 split the stump, insert into the cleft the wedge-shaped end 

 of the scion, and seal up the wound with wax or clay (Fig. 

 209). The cambiums of the stock and the scion must be 

 put into contact at some point; and hence it is usual to 

 insert a scion in each side of the cleft, since the cambium of 

 the stock is comparatively near the surface. The cambiums 

 of stock and scion unite, the wound heals, and the scion 



